Separation of dry matter from liquid is known in the art. Methods such as precipitation, centrifugation and filtering are commonly used for separation purposes in a vast number of industries. The latter separation method is relevant for the present invention.
Several problems concerning the separation of dry matter from liquid exist. In precipitation the particles in the liquid precipitate. Often a precipitation agent is added to the liquid. After precipitation the dry matter is situated on the bottom as sludge, wherein the sludge will still comprise a good deal of liquid, particularly in the area between the sludge and the liquid.
Centrifugation is a accelerated type of precipitation, where agents aiding the separation is also added. Centrifugation is a very expensive separation method.
When filtering liquids having a high dry matter content there are significant problems concerning accumulation of dry matter on the filter. This accumulation is known as the filter cake. In conventional methods the filter cake will grow until further filtering is impossible and the filter then has to be cleaned. There are various techniques for limiting the filter cake. One such technique is cross-flow. Here, the suspension (feed stream) is made to move along the filter in such a way that the filter cake is forced to move along by the stream.
Another filtering method is back flushing. Here, the movement of the suspension is reversed to lift the filter cake from the filter.
When using the cross-flow or back flushing methods the filter cake is not accumulated on the filter but is accumulated in the suspension. This requires the filtering process to be either stopped or for the existence of an outlet for the accumulated dry matter. In the latter situation the removed dry matter will still have a relatively high liquid content.
A further filtering method is flushing. The filtering process is stopped and the filter is washed. Here, the dry matter is accompanied by a lot of liquid.
In U.S. Pat. No. 4,310,424 suspended particles from waste suspensions are removed by the means of a rotating drum barrel under pressure. Bigger particles will be deposited on a polymer network and smaller particles will be retained in a hydrophobic foam.
In “Filtration and Separation”, 1988 (improving cake de-watering) a system for the removal of water from sludge is described. The sludge is led pass a transport convey belt for partial dehydration. Hereafter, the sludge is led between 2 layers of synthetic filters, which in a continuing process is pressed through rollers. The first transport convey belt is not passed through the sludge suspension.
Nalco Chemical Company (1985) is disclosing a system for the dehydration of sludge from steel production by a twin-belt-filtering method. Sludge is pressed through two filter belts by the means of a pair of rollers. Two filter belts are used, wherein none is passed through the sludge suspension.
The above problems are overcome by the present invention by presenting a method and an apparatus capable of continuously removing dry matter from a liquid having a high dry matter content, and thereby provide a method and apparatus capable of reducing the loss of a liquid from the suspension and at the same time having an increased filtering capacity and a economical advantage over the prior art.